In atomic power plants or thermoelectric power plants, austenitic stainless steel pipes (hereinafter referred to as "stainless steel pipes") are often laid out in the form of one piping system constructed by welding and connecting straight pipes, curved pipes and joint pipes. In piping systems of this type, occurrence of damages so-called "stress corrosion cracks" is often observed in the vicinity of welded portion. It has been clarified that this stress corrosion crack is due to the tensile stress caused by thermal influences at the step of welding stainless steel pipes and left in the inner faces of the pipes, sensitizaton of the steel structure in the pipes and other factors combined therewith. Accordingly, there have been proposed various methods in which causes of the stress corrosion crack owing to thermal influences at the welding step are eliminated. For example, there can be mentioned a method in which the residual tensile stress left in the inner face of a pipe, which is one of causes of the stress corrosion crack, is improved and eliminated. According to this conventional method, as shown in FIG. 1, while the interior of a stainless steel pipe 1 is being cooled by a coolant 2 such as water, a high frequency induction heating element 4 (hereinafter referred to as "induction element") is applied to the outer side of the vicinity 3 of the welded portion 3a of the steel pipe 1 so that the welded portion 3a is located substantially at the center of the heating width of the induction element 4 and the vicinity 3 of the welded portion is induction-heated from the outside, whereby a thermal stress exceeding the yield point is generated in the heated portion while producing a sufficient temperature difference between the inner face 11 and outer face 10 of the steel pipe 1. Then, the heated portion is cooled naturally to the ambient temperature to eliminate the temperature difference between the inner and outer faces of the pipe.
In this known method, since the vicinity 3 of the welded portion of the stainless steel pipe 1 is located substantially at the center in the widthwise direction of the induction element, a sufficient temperature difference can be produced between the inner and outer faces in this pipe 1 at the heating step and it is possible to produce a compression yield in the outer face 10 of the pipe and a tensile yield in the inner face 11 of the pipe. Accordingly, if this temperature difference is eliminated by stopping the heating, the portion which has undergone the compression yield in the outer face of the pipe is pulled and the portion which has undergone the tensile yield in the inner face of the tube is compressed. As a result, the tensile residual stress is left in the outer face 10 of the pipe and the compressive residual stress is left in the inner face 11 of the pipe. Accordingly, in the inner face 11 where the tensile stress given by thermal influences at the welding step is left, an effect of the previous residual stress is moderated or shifting the previous residual stress to the compression side. However, it has been found that a serious difficulty is involved in this method.
More specifically, when the above method is applied to a piping system which has already been laid out in an atomic power plant or the like, it is not always possible to locate the vicinity of the welded portion, where the residual stress is to be improved, substantially at the center in the widthwise direction of the induction element. In those cases, the portion where improvement of the residual stress is required is not sufficiently heated and it is impossible to produce a temperature difference sufficient to improve the residual stress between the inner and outer faces of the heated portion.
For example, if the welded portion 3a is a T-shaped branched portion of the piping system as shown in FIG. 2, since other pipe crossing the pipe having the welded portion 3a gets in the way, the welded portion 3a cannot be located substantially at the center of the heating width of the induction element 4. Therefore, even if heating is carried out in this state, the heating temperatures on the inner and outer faces of the vicinity 3 of the welded portion and the temperature differences are as shown by a temperature distribution curve of FIG. 2 and it is impossible to provide heating temperatures and temperature differences sufficient to improve the residual stress in the inner and outer faces of the pipe.
This disadvantage is due to a characteristic property of an ordinary induction heating element that a high heat generation density is obtained substantially at the center in the widthwise direction thereof.
Accordingly, it may be expected that if an induction element capable of providing a high heat generation density on each of both the side ends in the widthwise direction thereof, the above-mentioned difficulty will be overcome and eliminated. However, it is actually impossible to sufficiently eliminate the above difficulty only by using a conventional induction element which is arranged so that a heat generating density on a material located on one side end portion in the widthwise direction of the induction element can be increased.